1. Field of the Invention
The present invention generally relates to a driving-force regulating apparatus for an electric vehicle which is driven by an electric motor and particularly to such an apparatus which is capable of regulating the output torque of the electric motor to a creep torque so that the electric vehicle creeps, i.e., is driven at a very low speed around zero.
2. Related Art Statement
There is known an electric vehicle which is driven by an electric motor in such a way that the output torque of the motor is regulated to a running torque corresponding to (a) an amount of operation of the accelerator pedal of the vehicle and (b) a current speed of rotation of the motor upon operation of the accelerator pedal. When the operation amount of the accelerator pedal is zero, the output torque of the electric motor is regulated to zero. Unlike an AT (automatic-transmission) vehicle having a hydraulic torque converter, the conventional electric vehicle is not capable of producing a creep torque. Therefore, when the driver or operator starts the electric vehicle on an upgrade road surface, he or she must depress the accelerator pedal immediately after releasing the brake pedal, or use the side brake. Otherwise, the electric vehicle will be dragged backward or downward on the upgrade road surface. Thus, operating the electric vehicle is considerably difficult for those accustomed to operating the AT vehicle.
In the technical background, Japanese Patent Application laid open for public inspection under Publication No. 3(1991)-253202 discloses the art of regulating, when an electric vehicle is stopped on an upgrade road surface, the output torque of an electric motor to a creep torque corresponding to the degree of upgrade of the road surface detected by a sensor, so that the vehicle is not dragged backward on the upgrade road surface. The output torque of the electric motor may be regulated to a creep torque corresponding to an operation amount of a manually operable member available to the operator. Thus, the driving-force regulating apparatus disclosed by the Japanese document is capable of effecting a creep torque regulation.
The above driving-force regulating apparatus is adapted to regulate, when the operator operates the accelerator pedal for starting the electric vehicle on the upgrade road surface following the creep torque regulation, the output torque of the electric motor again to a normal or running torque, according to a pre-stored data map indicative of a relationship between (1) motor output torque and (2) parameters, i.e., (2a) operation amount of the accelerator pedal and (2b) rotation speed of the electric motor. However, the data map is prepared mainly for the case where the vehicle is driven on a flat road surface. Therefore, in the case where the vehicle is started on a highly upgrade road surface, the operator must continue to fully depress the accelerator pedal so that the electric motor produces a sufficiently great driving force to start the vehicle on the highly upgrade road surface. Thus, the operability of the electric vehicle is not satisfactory.
A backward dragging force exerted to a specific electric vehicle on an upgrade road surface differs from those exerted to other vehicles, because the vehicles have different weights due to different equipments and/or different numbers of passengers, and exhibit different rolling resistances of tires. Additionally, since parts of the specific vehicle are deteriorated and/or worn because of use or service, the backward dragging force exerted to the vehicle is subject to time-wise change. Therefore, if the vehicle is driven on the same upgrade road surface with the same creep torque after a certain period of use, the vehicle might be dragged backward. If the vehicle is adapted, by taking into consideration the individuality and time-wise change of the vehicle, such that the output torque of the electric motor is regulated to a greater than necessary creep torque, the backward dragging of the vehicle would be prevented but the amount of consumption of electric power would be more than necessary. Even if where the output torque of the electric motor may be regulated to a creep torque selected by the operator, it is, in fact, impossible to select a creep torque appropriately corresponding to the individuality and one or more current conditions of the vehicle.
Japanese Patent Application laid open for public inspection under Publication No. 2(1990)-65604 discloses the art of producing, when the running speed of an electric vehicle is around zero with the accelerator pedal being released (i.e., "OFF"), an output torque of the electric motor in a direction opposite to a direction corresponding to the direction of movement of the vehicle, so that the vehicle is prevented from being dragged forward or backward on a downgrade or upgrade road surface, respectively.
Since, however, the above-indicated, second driving-force regulating apparatus is adapted to increase the output torque of the electric motor based on the distance of movement of the electric vehicle, the vehicle is subjected to repetitive forward and backward movements on the downgrade or upgrade road surface. Thus, the ride comfort of the vehicle is lowered. Additionally, as the degree of downgrade or upgrade of the road surface increases, or as the weight of the electric vehicle increases due to, e.g., increased number of passengers, the distance of movement of the vehicle increases. Moreover, the second apparatus controls the electric motor to produce an output torque in an opposite direction, also when the running speed of the electric vehicle is reduced to around zero as a result of operation ("ON") of the brake pedal. Thus, this arrangement leads to wasteful consumption of electric power.
The above-indicated, first driving-force regulating apparatus is adapted to regulate the output torque of the electric motor to a creep torque, independently of change of steering angle of the electric vehicle. When the steering angle is changed by rotating the steering wheel, the running resistance of the vehicle is changed and accordingly the running speed of the vehicle is changed. Therefore, if the electric vehicle is driven with a great steering angle for, e.g., putting the vehicle into a garage, the vehicle might be stopped because of an increased running resistance. For avoiding this, it is possible to adapt the first apparatus to regulate the output torque of the electric motor to a greater creep torque. In this case, however, when the electric vehicle is driven with a small steering angle, the vehicle would be driven at a higher than necessary running speed. Thus, the operability of the vehicle would not be improved but lowered. Although the operator can use the manually operable member to adjust the creep torque, it is considerably difficult to adjust the creep torque while simultaneously operating or rotating the steering wheel. Therefore, it is, in fact, impossible to obtain an appropriate creep torque corresponding to the current steering angle of the vehicle.